This invention relates generally to telephone voice communication systems, and, more particularly, to systems that multiplex together both voice and digital data signals.
Typical telephone voice communication systems include a great number of remote users, each with its own remote telephone unit interfacing with a dedicated two-wire, full-duplex line. A central office apparatus receives the analog voice signal transmitted from each user and transmits back a corresponding analog voice signal over the same full-duplex line. The central office apparatus includes an analog switch array for routing the individual voice signals to their desired destinations.
Many such telephone voice communication systems include intermediate digital loop carrier (DLC) units that group together the analog voice signals received from a number of separate remote users and time-division multiplexes these signals onto a single carrier, for transmission over a relatively long distance to the system's central office. A corresponding DLC unit located at the central office demultiplexes this signal to recover the original analog voice signal for each user. These two DLC carrier units perform the same functions for voice signals traveling in the opposite direction, i.e., toward the various remote users.
Interest has recently been expressed in adapting each remote user's equipment to transmit simultaneously both a voice signal and a digital data signal over a common full-duplex line to the central office. The voice and data signals are separated there and routed to separate destinations. There particularly has been expressed an interest in transmitting two digital data signals along with the voice signal from each user. One such digital data signal would have a relatively low data rate suitable, for example, for carrying signaling information for water meters, burglar and fire alarms and the like. The other data signal would have a higher data rate suitable, for example, for transmitting data to and from a personal computer or the like.
The usual technique for simultaneously transmitting both voice and digital data is to transmit the signals on different lines or to frequency-division multiplex them together on a single line. In the latter case, the voice signal is ordinarily retained in its analog format, with a limited bandwidth, and the two digital data signals are modulated onto different subcarriers at frequencies spaced above the voice signal. Although such a system is generally effective at simultaneously transmitting and receiving both voice and digital data signals, it has not been proven to be entirely satisfactory. The quality of the voice signal is unduly distorted by the band limiting, and the upper limits on the data rates of the two digital data signals is considered to be unduly low.
It should therefore be appreciated that there is a need for a communication system that can transmit and receive both voice and digital data signals without any undue distortion to the voice signal and without any undue limits on digital data rates. The present invention fulfills this need.